Production of polyoxymethylene and suitable catalysts

ABSTRACT

A process is described for preparing polyoxymethylene by contacting a formaldehyde source with a catalyst of the formula I 
 
[ML 1   a L 2   b ] c   m+ Z c·m/n   n−   (I) 
where M is a metal of group VIII; 
         L 1  is a ligand having at least one π-electron pair;    each L 2  is independently tetrahydrofuran or a ligand which is displaceable by tetrahydrofuran; Z is an anion; a is 1 or 2; b is an integer from 0 to 4; c is 1 or 2; and m and n are integers from 1 to 4.

The present invention relates to a process for preparingpolyoxymethylene by contacting a formaldehyde source with a catalyst anda catalyst suitable therefor.

The polyoxymethylene resulting from the homopolymerization offormaldehyde is a polymer having repeating CH₂O units. When formaldehydeis copolymerized with cyclic ethers or formals, the CH₂O chains areinterrupted by units which stem from the cyclic ethers or formals. Theterm polyoxymethylene is used hereinbelow both for the homo- and for thecopolymer.

Polyoxymethylene and processes for preparing it by homo- orcopolymerizing formaldehyde using metal complexes as catalysts are wellknown. For instance, WO 94/09055 describes the polymerization of cyclicethers, such as trioxane, in the presence of a catalyst of the generalformula MZ₂Q_(t), where M is a metal, at least one Z is a perfluorinatedalkylsulfonate and any further Z moieties present are each oxo or amonovalent monoanion, Q is a neutral ligand, s is from 2 to 5 and t isfrom 0 to 6. Specifically, the polymerization of trioxane in thepresence of ytterbium triflate is described. However, the unsatisfactoryyields even at long reaction times are disadvantageous.

U.S. Pat. No. 3,457,227 describes trioxane homopolymerization andcopolymerization with cyclic ethers using a dioxomolybdenumacetylacetonate catalyst. A disadvantage is that the catalyst is easilydeactivated by impurities or water traces in the trioxane. The trioxaneused accordingly has to be very pure.

DE 2 226 620 describes the polymerization of formaldehyde using a copperacetylacetonate complex. This also requires virtually water-freeformaldehyde.

U.S. Pat. No. 3,305,529 describes the homo- and copolymerization offormaldehyde in the presence of metal diketonates. However, the yieldsobtained are unsatisfactory for an industrial process.

BE 727 000 describes the homopolymerization of formaldehyde or trioxaneand the copolymerization of cyclic formals with a catalyst whichcomprises titanyl acetylacetonate and iron(II) and/or iron(III)acetylacetonate. This also requires the monomers used to besubstantially water-free.

The prior art processes have long induction times, in particular whenthe formaldehyde source is not highly pure. This may even lead to nopolymerization occurring at all. The induction time is the time whichelapses from the mixing of the formaldehyde source with the catalyst tothe “light-off” of the polymerization. A long induction time leads tolong residence times of reactants in the reactor which is uneconomical.

It is an object of the present invention to provide a process having ashort induction time which is preferably tolerant toward impurities andwater traces in the formaldehyde source. In particular, the catalystused in the process shall be light and recycleable without substantialloss of activity.

We have found that this object is achieved by a process for preparingpolyoxymethylene by contacting a formaldehyde source with a catalyst ofthe formula I[ML¹ _(a)L² _(b)]_(c) ^(m+)Z_(c·m/n) ^(n−)  (I)where

-   -   M is a metal of group VIII of the Periodic Table;    -   L¹ is a ligand having at least one π-electron pair;    -   L² is tetrahydrofuran or a ligand which is displaceable by        tetrahydrofuran;    -   Z is an anion;    -   a is 1 or 2;    -   b is an integer from 0 to 4;    -   c is 1 or 2; and    -   m and n are integers from 1 to 4.

In formula I, M is preferably Co, Rh, Ir, Ni, Pd or Pt. M is morepreferably Ir(III) or Pt(II).

L¹ is a ligand which has at least one π-electron pair capable ofcomplexing, preferably at least two π-electron pairs capable ofcomplexing. The ligand may be charged or uncharged. The π-electron pairsmay be the π-contribution of an element-element double bond, preferablyof a carbon-carbon double bond, or a free electron pair which isconjugated with at least one element-element double bond, for example inthe allylic position to such a double bond, and preferably localized ona carbon atom. The useful L¹ ligands include olefins such as conjugatedor nonconjugated, cyclic or open-chain dienes, for example butadiene,cyclopentadiene, cycloheptadiene or cyclooctadiene, trienes andtetraenes, aromatic compounds such as benzene and cyclically conjugatedcarbanions or carbocations having aromatic character, i.e. those whichhave 6 π-electrons or one cyclically conjugated system, such as thecyclobutene dianion, the cyclopentadienyl anion or the cycloheptatrienylcation, and also the allyl anion.

The olefins mentioned, aromatic compounds (including the compoundshaving aromatic character) and the allyl anion may each carrysubstituents on one, more than one or all of their carbon atoms, such asin particular alkyl, alkenyl, aryl, heteroaryl, aralkyl, COOR², COR², CNor NO₂ where R² is H, alkyl, aryl or aralkyl.

For the purposes of the present invention, the term “alkyl” encompasseslinear, branched and cyclic alkyl groups. These are preferablyC₁-C₂₀-alkyl, in particular C₁-C₆-alkyl groups, such as methyl, ethyl,propyl, isopropyl, n-butyl, 2-butyl, isobutyl, tert-butyl, n-pentyl andn-hexyl, or C₃-C₈-cycloalkyl, such as cyclopropyl, cyclopentyl,cyclohexyl or cycloheptyl.

Aryl is preferably C₆-C₁₄-aryl, such as phenyl, naphthyl, anthracenyl orphenanthrenyl and in particular phenyl or naphthyl. The aryl radicalsmay carry up to three C₁-C₄-alkyl radicals.

Heteroaryl is preferably a radical of a 5- or 6-membered heteroaromatichaving from 1 to 5 heteroatoms selected from N, O and S, such aspyrrole, furan, thiophene, pyrazole, oxazole, pyridine, pyrimidine,pyridazine or pyrazine. The heteroaryl radicals may be benzofused alongone or two bonds.

Aralkyl is preferably C₇-C₂₀-aralkyl, such as benzyl or phenylethyl.

The term “alkenyl” encompasses linear, branched and cyclic alkenylgroups. These are preferably C₂-C₂₀-alkenyl groups, in particularC₂-C₆-alkenyl groups, such as ethenyl, propenyl, isopropenyl, n-butenyl,isobutenyl, n-pentenyl and n-hexenyl, or C₅-C₈-cycloalkenyl, such ascyclopentenyl, cyclohexenyl, cycloheptenyl or cyclooctenyl.

Preferred L¹ ligands are cyclic dienes which may be substituted asstated, and particular preference is given to cyclooctadiene andcyclopentadienyl anions of the formula C₅H_((5-u))R¹ _(u) where u is aninteger from 0 to 5 and each R¹ is independently one of the substituentsstated above. A particularly suitable cyclopentadienyl anion is thepentamethylcyclopentadienyl anion.

The L² ligands are tetrahydrofuran (THF) or a ligand which, owing to thehigh affinity of THF for the central atom, is displaceable from thecoordination sphere of a complex by THF. The displaceability of a ligandby another coordinates generally with their positions in thespectrochemical series of ligands, so that useful L² ligands are thosewhich lead to a smaller ligand field splitting than THF. A ligand isregarded as displaceable by THF when it has been displaced from acomplex by heating the complex in THF to boiling. A suitable modelsystem which can be used to study the displaceability of a ligand by THFis dichlorohexakis-(ethanol)nickel(II). This is firstly reacted with theligand to be tested to convert it into a complex which contains at leastone molecule of this ligand in coordinatively bound form. It isgenerally sufficient to stir the dichlorohexakis(ethanol)-nickel(II)with an excess of ligand to be tested in ethanol at room temperature.The complex obtained may then be isolated and heated as described in THFto boiling. A ligand is regarded as displaceable by THF when at leastone molecule is displaced by THF from a complex which contains more thanone molecule of a ligand. The L² ligands are preferably neutral. When cis 1 in the catalyst used according to the invention, b is preferablynot 0.

Preference is given to selecting L² from nitriles, CO, alkenes, aminesdisplaceable by THF, ethers displaceable by THF, carboxylic esters,cyclic carbonic esters, epoxides, hemiacetals, acetals and nitrocompounds.

The term “nitrile” encompasses in particular compounds of the generalformula R³CN, where R³ is an optionally halogenated alkyl, aryl oraralkyl radical. R³ is more preferably methyl, ethyl, propyl, isopropyl,n-butyl, isobutyl, sec-butyl or tert-butyl.

Examples of useful nitriles include acetonitrile, propionitrile andbenzonitrile.

Amines displaceable by THF are in particular aromatic amines and amineshaving a sterically shielded nitrogen atom. Examples of useful aminesinclude diisopropylamine, N,N-dimethylaniline and diphenylamine.

Ethers displaceable by THF are in particular both open-chain ethershaving electron-withdrawing and/or sterically demanding radicals andalso cyclic ethers. The preferred open-chain ethers include diphenylether and methyl tert-butyl ether. Preferred cyclic ethers aretetrahydrofuran and 1,4-dioxane.

Carboxylic esters encompass in particular compounds of the generalformula R⁴COOR⁵, where R⁴ and R⁵ are each independently as defined forR³. R⁴ may also be H. R⁴ and R⁵ may also form a bridging unit. R⁴ and R⁵are preferably each independently methyl, ethyl, propyl, isopropyl,n-butyl or phenyl. Examples of useful carboxylic esters include methylacetate and ethyl acetate.

Cyclic carbonic esters encompass in particular compounds of the generalformula R⁶OCOOR⁷ where R⁶ and R⁷ together form a C₂-C₄-alkylene bridgewhich may be partly or fully halogenated or carry from 1 to 4 alkylradicals. Examples of useful cyclic carbonic esters include ethylenecarbonate and propylene carbonate.

Epoxides encompass in particular compounds of the general formula

where R⁸, R⁹, R¹⁰ and R¹¹ are each independently as defined for R³ orare H.

Examples of useful epoxides include ethylene oxide, propylene oxide andbutylene oxide.

Hemiacetals and acetals encompass in particular compounds of the generalformula R¹²OCR¹³R¹⁴OH and R¹²OCR¹³R¹⁴OR¹⁵, where R¹², R¹³, R¹⁴ and R¹⁵are each independently as defined for R³, and R¹³ and R¹⁴ may also be Hor together form a C₂-C₇-alkylene bridge, and R¹² and R¹⁵ may also forma C₂-C₄-alkylene bridge which may be interrupted by one or two oxygenatoms. Examples of useful acetals include trioxane, 1,3-dioxane,1,3-dioxepane and cyclopentanone dimethylacetal.

Nitro compounds encompass compounds of the general formula R¹⁶NO₂, wereR¹⁶ is as defined for R³. Examples of useful nitro compounds includenitromethane and nitrobenzene.

Particular preference is given to selecting the ligands L² from THF andCO.

Z is one or more anions which may be identical or different. Each Z ispreferably an anion derived from a Brönsted acid whose pK_(a) is smallerthan that of acetic acid or a noncoordinating anion. The term“noncoordinating anion” is known to those skilled in the art. These areanions where the charge is effectively distributed over more than oneatom so that there are no point-centered charges. Z is more preferably ahalide, in particular chloride, a sulfonate of the general formulaROSO₂—, where R is alkyl, partly or fully halogenated alkyl or aryl,such as trifluoromethanesulfonate, benzenesulfonate orp-toluenesulfonate, a carboxylate of the general formula R′COO—, whereR′ is as defined for R and more preferably fully halogenated alkyl, inparticular perfluorinated alkyl, such as trifluoroacetate, a complexedborate such as tetrafluoroborate or tetraphenylborate, a complexedphosphate such as hexafluoro-phosphate, a complexed arsenate such ashexafluoroarsenate or a complexed antimonate such as hexafluoro- orhexachloroantimonate, with the proviso that not all Z radicals may behalide. In particular, at least one Z radical is perfluoroalkylsulfonatesuch as trifluoromethanesulfonate, tetrafluoroborate,hexafluorophosphate or hexafluoroantimonate.

b is an integer from 0 to 4 and depends on the maximum possiblecoordination number of the central metal.

c is 1 or 2, i.e. the complex I may be either mononuclear or binuclear.The two metal centers in a binuclear complex are bridged by at leastone, preferably at least two bridges which are formed by L² and/or Z.

m is an integer from 1 to 4 and results from the sum total of theoxidation number of M and the charges of the L¹ and L² ligands. n is thecharge of the anion Z.

Preferred catalysts are[Pd(II)(cod)(THF)_(x)](SbF₆)₂,[Pd(II)(cod)(CH₃CN)_(x)](PF₆)₂ and[Ir(III)Cp*Cl₂Ir(III)Cp*Cl]CF₃SO₃

-   -   where    -   cod is cyclooctadiene,    -   THF is tetrahydrofuran,    -   Cp* is pentamethylcyclopentadienyl and    -   x is an integer from 1 to 3.

The catalyst I is preferably used in a quantity of from 1 ppm to 1 mol%, more preferably from 5 to 1000 ppm and in particular from 50 to 500ppm, based on the formaldehyde source.

Preference is given to preparing the catalyst I before use in thepolymerization. The catalyst is prepared by customary processes forpreparing such metal complexes and the preparation is familiar to thoseskilled in the art.

The formaldehyde source used is preferably formaldehyde, trioxane,tetraoxane or paraformaldehyde or a mixture thereof, and more preferablyformaldehyde or trioxane or a mixture thereof. Trioxane, the cyclictrimer of formaldehyde, and paraformaldehyde, an oligomer having from 2to 100 formaldehyde units, are either depolymerized before use in thepolymerization reaction or preferably used as such and dissociated inthe course of the reaction.

The formaldehyde source preferably has a degree of purity of at least95%, more preferably at least 98% and most preferably at least 99%. Inparticular, the formaldehyde source contains a maximum of 0.002% byweight of compounds having active hydrogen such as water, methanol orformic acid, based on the weight of the formaldehyde source. However,the process according to the invention also tolerates formaldehydesources having a lower degree of purity and a higher content ofcompounds having active hydrogen.

The process according to the invention may be carried out as a solution,suspension, gas phase or bulk polymerization.

When the polymerization is carried out in solution or suspension, it isadvantageous to select a substantially anhydrous aprotic organicreaction medium which is liquid under the reaction conditions and reactsneither with the catalyst nor with the formaldehyde source. When thepolymerization is carried out in solution, the solvent shouldadvantageously also dissolve the catalyst and the formaldehyde sourcebut preferably not dissolve or only sparingly dissolve thepolyoxymethylene formed. When the polymerization is carried out insuspension, the formaldehyde source should also be insoluble in thesolvent and, if necessary, dispersion auxiliaries are used, in order toachieve better distribution of the formaldehyde source in the reactionmedium.

Preference is given to selecting the solvent from saturated orunsaturated, linear or branched, aliphatic hydrocarbons which may bepartly or fully halogenated, optionally substituted alicycles,optionally substituted fused alicycles, optionally substitutedaromatics, acyclic and cyclic ethers, polyether polyols and other polaraprotic solvents such as sulfoxides and carboxylic acid derivatives.

Examples of useful aliphatic hydrocarbons include propane, n-butane,n-pentane, n-hexane, n-heptane, n-decane and mixtures thereof. Examplesof useful halogenated hydrocarbons include methylene chloride,chloroform, carbon tetrachloride, dichloroethane or trichloroethane.Useful aromatics include benzene, toluene, the xylenes, nitrobenzene,chlorobenzene, dichlorobenzene and biphenyl. Useful alicycles includecyclopentane, cyclohexane, tetralin and decahydronaphthalene. Examplesof useful acyclic ethers include diethyl ether, dipropyl ether,diisopropyl ether, dibutyl ether and butyl methyl ether; useful cyclicethers include tetrahydrofuran and dioxane. Examples of the usefulpolyether polyols include dimethoxyethane and diethylene glycol. Anexample of a useful sulfoxide is dimethyl sulfoxide. The usefulcarboxylic acid derivatives include dimethylformamide, ethyl acetate,acetonitrile, acrylate and ethylene carbonate.

Particularly preferred solvents for the solution polymerization areselected from the following: n-hexane, cyclohexane, methylene chloride,chloroform, dichloroethane, trichloroethane, tetrachloroethane, benzene,toluene, nitrobenzene, chlorobenzene, dichlorobenzene, tetrahydrofuranand acetonitrile. All mixtures thereof are also suitable. In particular,1,2-dichloethane or 1,2-dichloroethane are used in a mixture withhexane, cyclohexane or benzene.

Preference is given to using the formaldehyde source in the solutionpolymerization in a concentration of from 20 to 90% by weight,preferably from 30 to 80% by weight, based on the total weight of thesolution. The polymerization in solution may also be carried out as a“blow-in” polymerization. This involves continuously blowing theformaldehyde source, in particular formaldehyde gas, into a solutionwhich contains the catalyst.

Preferred reaction media for the heterogeneous suspension polymerizationinclude straight-chain aliphatic hydrocarbons.

The polymerization may also be carried out in bulk when trioxane is usedas the formaldehyde source. Trioxane is used as a melt; the reactiontemperature and reaction pressure are selected correspondingly.

In the process according to the invention, the sequence in which theformaldehyde source and the catalyst I introduced into the reaction zoneis not of decisive importance. However, preference is given to initiallycharging the formaldehyde source and adding the catalyst to it.

The polymerization is preferably carried out at a temperature of from−40 to 150° C., more preferably from 0 to 150° C. The solutionpolymerization and suspension polymerization are carried out inparticular at from 20 to 100° C. and especially from 30 to 90° C. Thebulk polymerization is preferably carried out at such a temperature thatthe formaldehyde source, especially trioxane, and the polymer are in theform of a melt. In particular, the temperature, depending on thepressure, is from 60 to 120° C., especially from 60 to 100° C.

The reaction pressure is preferably from 0.1 to 50 bar, more preferablyfrom 0.5 to 10 bar and in particular from 1 to 5 bar.

Useful reaction apparatus includes the reactors which are known to theskilled in the art for the type and conditions of each differentpolymerization.

The above remarks apply both to the homopolymerization of theformaldehyde source and to the copolymerization of the formaldehydesource with cyclic ethers or formals which will be referred tohereinbelow as comonomers.

Homopolymeric polyoxymethylene tends to thermally degrade, i.e. todepolymerize to oligomeric or monomeric formaldehyde. This is attributedto the presence of hemiacetal functions at the chain ends of thepolyoxymethylene. Copolymerization of formaldehyde with comonomers suchas cyclic ethers and/or formals can stabilize the polyoxymethyleneformed. These comonomers are incorporated in the polyoxymethylene chain.When the polymer is subjected to thermal stress, the polyoxymethylenechain degrades until the chain end is formed by one of theabovementioned comonomers. These are substantially less prone tothermally degrade, so that the depolymerization comes to a stop and thepolymer is stabilized. Useful comonomers of this type are cyclic ethers,in particular those of the formula

where R^(a), R^(b), R^(c) and R^(d) are each independently hydrogen oran optionally halogenated C₁-C₄-alkyl group, R^(e) is a —CH₂—, —CH₂O—, aC₁-C₄-alkyl- or C₁-C₄-haloalkyl-substituted methylene group or acorresponding oxymethylene group and n is an integer from 0 to 3.

Cyclic ethers mentioned only by way of example include ethylene oxide,1,2-propylene oxide, 1,2-butylene oxide, 1,3-butylene oxide,1,3-dioxane, 1,3-dioxolane and 1,3-dioxepane, and comonomers mentionedonly by way of example include linear oligo- and polyformals such aspolydioxolane and polydioxepane.

When they are used, repeat units of the formula

are incorporated into the polyoxymethylene copolymers obtained inaddition to the —CH₂O— repeat units stemming from the formaldehydesource.

If desired, a third monomer may be used in addition to theabove-described cyclic ethers, preferably a bifunctional compound of theformula

where Z is a chemical bond, —O—, —ORO— (R=C₁-C₈-alkylene orC₂-C₈-cycloalkylene).

To name only a few examples, preferred monomers of this type includeethylene diglycide, diglycidyl ethers and diethers made from glycidyleneand formaldehyde, dioxane or trioxane in a molar ratio of 2:1 and alsodiethers made from 2 mol of glycidyl compound and 1 mol of an aliphaticdiol having from 2 to 8 carbon atoms, for example the diglycidyl ethersof ethylene glycol, 1,4-butanediol, 1,3-butanediol,cyclobutane-1,3-diol, 1,2-propanediol and cyclohexane-1,4-diol.

Particular preference is given to using ethylene oxide, 1,2-propyleneoxide, tetrahydrofuran, 1,3-dioxane, 1,4-dioxane, 1,3-dioxolane and1,3-dioxepane, in particular 1,3-dioxepane as comonomers.

The comonomers are preferably used in a quantity of from 0.1 to 40% byweight, more preferably from 0.2 to 10% by weight, in particular from0.5 to 5% by weight, based on the formaldehyde contained in theformaldehyde source.

The comonomers may either be initially charged with the formaldehydesource or added to the additionally charged catalyst together with theformaldehyde source. Alternatively, they may be added to the reactionmixture consisting of the formaldehyde source and the catalyst.

When cyclic ethers are used as comonomers, there is a risk that thesecontain peroxides, in particular when they have been stored for arelatively long time before use. Peroxides firstly lengthen theinduction time of the polymerization and secondly reduce the thermalstability of the polyoxymethylene formed owing to their oxidativeeffect. For this reason, preference is given to using cyclic etherswhich contain less than 0.0015% by weight, more preferably less than0.0005% by weight, of peroxides, reported as hydrogen peroxide and basedon the quantity of cyclic ether used.

In order to prevent oxidative degradation of the polyoxymethylenesobtained, preference is given to adding sterically hindered phenolantioxidants to them. In principle, useful sterically hindered phenolsinclude all compounds having a phenolic structure which have at leastone sterically demanding group on the phenolic ring.

Preference is given to using, for example, compounds of the formula

where R¹ and R² are identical or different and are each an alkyl group,a substituted alkyl group or a substituted triazole group and R³ is analkyl group, a substituted alkyl group, an alkoxy group or a substitutedamino group.

Antioxidants of the type mentioned are described, for example, in DE-A27 02 661 (U.S. Pat. No. 4,360,617).

A further group of preferred sterically hindered phenols is derived fromsubstituted benzenecarboxylic acids, in particular from substitutedbenzenepropionic acids.

Particularly preferred compounds from this class are compounds of theformula

where R⁴, R⁵, R⁷ and R⁸ are each independently C₁-C₈-alkyl groups whichmay themselves be substituted (at least one of them is a stericallydemanding group) and R⁶ is a bivalent aliphatic radical having from 1 to10 carbon atoms which may also have C—O-bonds in the main chain.

Preferred compounds of this type are

(Irganox® 245 from Ciba-Geigy)and

(Irganox® 259 from Ciba-Geigy)

Examples of sterically hindered phenols include:

-   2,2′-methylene-bis(4-methyl-6-tert-butylphenol),-   1,6-hexanediolbis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate]    (Irganox® 259), pentaerythrityl-   tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and the    above-described Irganox® 245

The following compounds have proven to be particularly effective and aretherefore used with preference:

-   2,2′-methylene-bis(4-methyl-6-tert-butylphenol),-   1,6-hexanediolbis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate]    (Irganox® 259), pentaerythrityl-   tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate],-   distearyl 3,5-di-tert-butyl-4-hydroxybenzylphosphonate,-   2,6,7-trioxa-1-phosphabicyclo[2.2.2]oct-4-yl-methyl-   3,5-di-tert-butyl-4-hydroxycinnamate,-   3,5-di-tert-butyl-4-hydroxyphenyl-3,5-distearyl-thiotriazylamine,-   2-(2′-hydroxy-3′-hydroxy-3′,5′-di-tert-butylphenyl)-5-chloro-benzotriazole,    2,6-di-tert-butyl-4-hydroxymethylphenol,-   1,3,5-trimethyl-2,4,6-tris-(3,5-di-tert-butyl-4-hydroxybenzyl)    benzene, 4,4′-methylene-bis(2,6-di-tert-butylphenol),-   3,5-di-tert-butyl-4-hydroxybenzyldimethylamine and-   N,N′-hexamethylene-bis-3,5-di-tert-butyl-4-hydroxyhydrocinnamide.

The sterically hindered phenols which may be used individually or as amixture may either be added to the monomer mixture or to the finishedpolymer. In the latter case, the polymer may optionally be melted inorder to achieve better dispersion of the antioxidant.

Preference is given to using the antioxidants in a quantity of up to 2%by weight, more preferably from 0.001 to 2% by weight, in particularfrom 0.005 to 1% by weight, based on the weight of monomer mixture usedor polymer obtained.

Another possible way of stabilizing the polyoxymethylene obtained byhomopolymerization of a formaldehyde source is to cap the hemiacetal endgroups. i.e. convert them to functionalities which do not tend tothermally degrade. To this end, the polyoxymethylene is reacted, forexample, with carboxylic acids, carbonyl halides, carboxylic anhydrides,carbonates or hemiacetals, or cyanethylated.

In this variant, the polyoxymethylene is stabilized in a separate stepfollowing the polymerization. Preference is accordingly given tostabilizing the polyoxymethylene by copolymerization with the comonomerswhich requires no separate step.

After the end of the polymerization reaction, preference is given toadmixing the catalyst with a deactivator. Useful deactivators includeammonia, aliphatic and aromatic amines, alcohols, basic salts such asalkaline metal and alkaline earth metal hydroxides and carbonates orborax, and also water. The deactivated catalyst and the deactivator arethen separated from the polymer, preferably by washing with water or anorganic solvent such as acetone or methylene chloride. However, sincethe catalyst I may also be used in very small quantities, subsequenttreatment of the polyoxymethylene to remove the catalyst may optionallyalso be omitted.

After the end of the polymerization reaction, excess monomer which isstill present in the reaction zone may be removed, for example, bydistillation, by purging with a gas stream, for example air or nitrogen,by degassing, by solvent extraction or by washing with an aqueousmixture or with an organic solvent such as acetone.

The polyoxymethylene is generally recovered by removing the solvent or,in the case of bulk polymerization, by cooling and optionally chippingthe melt. A preferred workup for bulk polymerization comprises thedischarge, cooling and chipping of the polymer melt at elevated pressureand in the presence of a liquid, in particular of water, and isdescribed in German patent application DE-A-100 06 037 which is fullyincorporated herein by way of reference.

In the process according to the invention, induction times are obtainedwhich are in the optimal range for industrial applications of from a fewseconds to a few minutes. At the same time, the catalyst quantityrequired is small. The polyoxymethylene preparable according to theinvention preferably has a number average molar mass M_(N) of from 5 000to 50 000 g/mol, more preferably from 10 000 to 30 000 g/mol. The weightaverage molar mass is preferably from 10 000 to 300 000 g/mol, morepreferably from 15 000 to 250 000 g/mol. The polydispersity index PDI(M_(W)/M_(N)) is preferably from 1.0 to 10.

The catalysts of the formula I used in the process according to theinvention are insensitive toward water present in the formaldehydesource and may easily be recycled without substantial losses ofactivity.

The invention is illustrated by the examples hereinbelow.

EXAMPLES

1. Preparation of the Catalysts

The catalysts were prepared under protective gas.

1.1 [Pd(II)(cod)(THF)_(x)](SbF₆)₂

20 mg (0.07 mmol) of PdCl₂(cod) were suspended in 2 ml of anhydrous THF.The suspension was suspended in a solution of 48 mg (0.14 mmol) ofAgSbF₆ in 2 ml of anhydrous THF and 0.1 ml of anhydrous acetonitrile.The mixture was stirred in the dark for 1 h and then precipitated AgClwas centrifuged off. After distillative removal of the solvent, theproduct was obtained as an orange-yellow oil.

1.2 [Ir(III)Cp*Cl₂Ir(III)Cp*Cl]CF₃SO₃

A solution of 46 mg (0.207 mmol) of trimethylsilyltrifluoromethanesulfonate in 5 ml of dichloromethane was added to asolution of 150 mg (0.188 mmol) of [IrCp*Cl₂]₂ in 20 ml ofdichloromethane at room temperature. After the reaction mixture had beenstirred for 1 day, the mixture was evaporated to dryness and the yellowresidue washed with hexane. The product was obtained in a yield of 161mg (0.177 mmol; 94% of theory) as a yellow powder.

2. Polymerization

The polymerizations were effected without protective gas.

2.1 Polymerization using [Ir(III)Cp*Cl₂Ir(III)Cp*Cl](CF₃SO₃)

2.1.1 Preparation of the Catalyst Solution

[Ir(III)Cp*Cl₂Ir(III)Cp*Cl](CF₃SO₃) is dissolved in the quantitiesstated in the individual experiments in a Schlenk flask in 1 ml ofdichloromethane. This solution is kept ready for the polymerization.

2.1.2 Polymerization

6 ml of liquid, distilled trioxane were admixed with 1 ml of thecatalyst solution prepared in 2.1.1 ({circumflex over (=)}7.7 μmol ofcatalyst) at 80° C. with stirring. The time from the addition of thecatalyst solution and the onset of cloudiness, i.e. the induction time,was 60 s. After 120 min, the polymer formed was filtered off, washedwith a little dichloromethane and dried. The yield was 4.48 g (68% oftheory). The polymer had an M_(n) of 21 300 g/mol, an M_(w) of 69 800g/mol and a PDI of 3.27.

2.1.3 Polymerization with Reuse of the Catalyst Solution from 2.1.2

The polymer filtrate from 2.1.2 was made up to a volume of 1 ml usingdichloromethane and used again for polymerization as described in 2.1.2instead of the catalyst solution prepared according to 2.1.1. Theinduction time was 10 s. The polymer was obtained in a yield of 80% withan M_(n) of 25 500 g/mol, an Mw of 30 000 g/mol and a PDI of 1.2.

2.1.4 Polymerization in the Presence of Water

A from 40 to 50% solution of trioxane in dichloroethane saturated withformalin was mixed with 3% by volume of 1,3-dioxepane. 6 ml of thissolution were admixed at 80° C. with 6.3 mg (6.9 μmol) of[Ir(III)Cp*Cl₂Ir(III)Cp*Cl]CF₃SO₃ in 1 ml of dichloromethane. Theinduction time was 25 min. After 1 h, a further 12 ml of the trioxanesolution were added and the mixture stirred for a further 15 h. Theresulting polymer was recovered by filtration in a yield of 10.3 g (76%of theory) having an M_(n) of 6 200 g/mol, an M_(w) of 15 000 g/mol anda PDI of 2.41.

2.2 Polymerization using [Pd(II)(cod)(THF)_(x)](SbF₆)₂

A solution of 0.025 mmol of [Pd(II)(cod)(THF)_(x)](SbF₆)₂ in 1 ml ofdichloroethane was added to 6 ml of the trioxane solution described in2.1.4 at 80° C. with stirring. The induction time was 70 S. After 30min, a further 6 ml of the trioxane solution was added and the inductiontime was this time 5 min. The resulting polymer was isolated by means offiltration in a yield of 6.25 g (91% of theory) having an Mn of 12 500g/mol, an M_(w) of 30 400 g/mol and a PDI of 2.42.

2.3 Comparative Experiments: Polymerization using MoO₂(acac)₂

2.3.1 Polymerization of Anhydrous Trioxane

6 ml of freshly distilled trioxane were admixed with a solution of 3 mg(9.15 μmol) of MoO₂(acac)₂ in 1 ml of dichloromethane at 80° C. withstirring. The time from the addition of the catalyst solution to theonset of cloudiness, i.e. the induction time, was 5.5 min. After 4 h,the polymer was filtered off, washed with a little dichloromethane anddried. The yield was 6.2 g (94% of theory), the M_(n) 11 500 g/mol, theM_(w) 28 400 g/mol and the PDI 2.48.

2.3.2 Polymerization in the Presence of Water

A solution of 9.5 mg (29 μmol) of MoO₂(acac)₂ in 1 ml of dichloromethanewas added to 6 ml of the trioxane solution described in 2.1.4 at 80° C.with stirring. After 2 h, there was still no polymerization. Thereaction was stopped.

1. A process for preparing polyoxymethylene by contacting a formaldehydesource with a catalyst of the formula I[ML¹ _(a)L² _(b)]_(c) ^(m+) Z_(c·m/n) ^(n−)  (I) where M is a metal ofgroup VIII; L¹ is cyclooctadiene; each L² isindependently•tetrahydrofuran or a ligand which is displaceable bytetrahydrofuran; Z is an anion; a is 1 or 2; b is an integer from 0 to4; c is 1 or 2; and m and n are integers from 1 to
 4. 2. A process asclaimed in claim 1 where M is Co, Rh, Ir, Ni, Pd or Pt.
 3. A process asclaimed in claim 1 where L² is selected from tetrahydrofuran, nitrites,CO, alkenes, amines, ethers, carboxylic esters, cyclic carbonic esters,epoxides, hemiacetals, acetals and nitro compounds.
 4. A process asclaimed in claim 3 where L² is selected from acetonitrile,tetrahydrofuran and CO.
 5. A process as claimed in claim 1 where Z is ahalide, sulfonate of the formula OSO₂R, where R is alkyl, partially orfully halogenated alkyl or aryl, carboxylate, complexed borate,complexed phosphate, complexed arsenate or complexed antimonate, withthe proviso that not all Z radicals are halide.
 6. A process as claimedin claim 5 wherein at least one Z radical is a perfluoroalkylsulfonate,tetrafluoroborate, hexafluorophosphate or hexafluoroantimonate.
 7. Aprocess as claimed in claim 1 where the catalyst is selected from[Pd(II)(cod)(THF)_(x)](SbF₆)₂ and[Pd(II)(cod)(CH₃CN)_(x)](PF₆)₂ where cod is cyclooctadiene, THF istetrahydrofuran and x is an integer from 1 to
 3. 8. A process as claimedin claim 1 where the formaldehyde source is formaldehyde, trioxane orparaformaldehyde.
 9. A process for preparing polyoxymethylene bycontacting a formaldehyde source with a catalyst of the formula[Ir (III)Cp*Cl₂Ir(III)Cp*Cl]CF₃SO₃ where Cp* ispentamethylcyclopentadienyl.